Rankine cycle system

ABSTRACT

A Rankine cycle system includes: an evaporator for heating a liquid-phase working medium with thermal energy of exhaust gas of an engine so as to generate a gas-phase working medium; a displacement type expander for converting the thermal energy of the gas-phase working medium generated by the evaporator into mechanical energy; and a Rankine controller for manipulating an amount of liquid-phase working medium supplied to the evaporator so that the temperature of the gas-phase working medium supplied from the evaporator to the expander coincides with a target temperature, and for manipulating the rotational speed of the expander by changing a load of the expander so that the pressure of the gas-phase working medium supplied from the evaporator to the expander coincides with a target pressure. Simultaneously with operating conditions of the engine shifting to a fuel supply suspension state or a low load state, the Rankine controller suspends supply of the liquid-phase working medium to the evaporator, and decreases the rotational speed of the expander as quickly as possible within a range in which the pressure of the gas-phase working medium does not exceed the target pressure.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2005-17102 filed on Jan. 25, 2005 the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a Rankine cycle system that includes an evaporator for heating a liquid-phase working medium with thermal energy of exhaust gas of an engine so as to generate a gas-phase working medium, and a displacement type expander for converting the thermal energy of the gas-phase working medium generated by the evaporator into mechanical energy.

2. Description of the Related Art

WO03/031775 discloses a Rankine cycle system of an expander in which a target temperature and a target pressure for steam at which the operating efficiency becomes a maximum is set; the steam temperature is controlled at the target temperature by manipulating the amount of water supplied to an evaporator, and the steam pressure is controlled at the target pressure by manipulating the rotational speed of the expander.

Also in the case where an automobile engine is provided with a Rankine cycle system, it is necessary to control the temperature and pressure of steam supplied from an evaporator to an expander of the Rankine cycle system at a target temperature and a target pressure. However, since fuel supply to the engine is cut when the automobile is decelerated and the output becomes zero, the engine repeatedly runs and stops at short time intervals. Furthermore, in the case where the engine is stopped when the automobile stops at an intersection and the engine is started when the automobile starts moving, the engine output first decreases to zero and then recovers.

In such a case, if the conventional control is carried out, as shown in FIG. 10, since supply of exhaust gas to the evaporator is suspended when the engine stops, the temperature of steam generated by the evaporator gradually decreases from the target temperature (ref. region a), and feedback control is carried out so that the amount of water supplied to the evaporator decreases corresponding to deviation from the target temperature. In this process, since the steam temperature at the entrance of the expander does not promptly decrease, the water supply amount does not promptly decrease, and as a result the steam temperature might decrease down to a saturation temperature (ref. region a).

Furthermore, control of steam pressure at the entrance of the expander is carried out by feedback control of the rotational speed of the expander corresponding to deviation of the steam pressure from the target pressure, but since the feedback control for decreasing the rotational speed of the expander is started only after the steam pressure decreases accompanying a decrease in the steam temperature, the timing with which the rotational speed of the expander is decreased is delayed and the steam pressure might greatly decrease from the target pressure (ref. region b).

Similarly, also when the engine is started and the output recovers to an original state, due to a response lag of feedback control, there is a problem that it takes more time for the steam temperature to recover to the target temperature (ref. region c), and it takes more time for the steam pressure to recover to the target pressure (ref. region d).

In this way, when stopping and starting of the engine are repeated, the steam temperature and the steam pressure at the entrance of the expander deviate greatly from the target temperature and the target pressure, leading to a problem that the operating efficiency of the expander deteriorates.

SUMMARY OF THE INVENTION

The present invention has been accomplished under the above-mentioned circumstances, and it is an object thereof to minimize decrease in the temperature and pressure of a gas-phase working medium of a Rankine cycle system when operating conditions of an engine shift to a fuel supply suspension state or a low load state.

In order to achieve the above-mentioned object, according to a first feature of the invention, there is provided a Rankine cycle system comprising: an evaporator for heating a liquid-phase working medium with thermal energy of exhaust gas of an engine so as to generate a gas-phase working medium; a displacement type expander for converting the thermal energy of the gas-phase working medium generated by the evaporator into mechanical energy; temperature control means for manipulating an amount of liquid-phase working medium supplied to the evaporator so that the temperature of the gas-phase working medium supplied from the evaporator to the expander coincides with a target temperature; and pressure control means for manipulating the rotational speed of the expander by changing a load of the expander so that the pressure of the gas-phase working medium supplied from the evaporator to the expander coincides with a target pressure; wherein, simultaneously with operating conditions of the engine shifting to a fuel supply suspension state or a low load state, the temperature control means suspends supply of the liquid-phase working medium to the evaporator, and the pressure control means decreases the rotational speed of the expander as quickly as possible within a range in which the pressure of the gas-phase working medium does not exceed the target pressure.

With the first feature, simultaneously with the operating conditions of the engine shifting to the fuel supply suspension state or the low load state, the temperature control means suspends supply of the liquid-phase working medium to the evaporator. Therefore, a drop in temperature of the gas-phase working medium from the target temperature can be minimized. Further, since the pressure control means decreases the rotational speed of the expander as quickly as possible within the range in which the pressure of the gas-phase working medium does not exceed the target pressure, a drop in pressure of the gas-phase working medium from the target pressure can be minimized. Thus, the efficiency of the expander is maintained at a high value.

According to a second feature of the present invention, simultaneously with recovery of the operating conditions of the engine from the fuel supply suspension state or the low load state, the temperature control means increases supply of the liquid-phase working medium to the evaporator, and the pressure control means increases the rotational speed of the expander.

With the second feature, since, simultaneously with recovery of the operating conditions of the engine from the fuel supply suspension state or the low load state, the temperature control means increases supply of the liquid-phase working medium to the evaporator, and the pressure control means increases the rotational speed of the expander. Thus, it is possible to make the temperature and the pressure of the gas-phase working medium quickly coincide with the target temperature and the target pressure, thereby minimizing any decrease in the efficiency of the expander.

The above-mentioned object, other objects, characteristics, and advantages of the present invention will become apparent from a preferred embodiment that will be described in detail below by reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 to FIG. 9 show one embodiment of the present invention;

FIG. 1 is a diagram showing the overall arrangement of a Rankine cycle system,

FIG. 2 is a layout diagram of the Rankine cycle system,

FIG. 3 is a control block diagram of temperature control means,

FIG. 4 is a detail of part A in FIG. 3,

FIG. 5 is a control block diagram of pressure control means,

FIG. 6 is a detail of part B in FIG. 5,

FIG. 7 is a time chart for explaining characteristics of change in the temperature and pressure of steam,

FIG. 8 is a graph showing the relationship between optimum steam temperature and maximum efficiency of an evaporator and an expander, and

FIG. 9 is a diagram showing a map in which a target steam pressure is looked up from steam energy and steam temperature.

FIG. 10 is a time chart for explaining characteristics of change in the temperature and pressure of steam in a conventional example.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows the overall arrangement of a Rankine cycle system R to which the present invention is applied. The Rankine cycle system R, which recovers thermal energy of exhaust gas of an engine E and converts it into mechanical energy, includes an evaporator 11, an expander 12, a condenser 13, and a water supply pump 14. The evaporator 11 heats water with the exhaust gas discharged by the engine E so as to generate high temperature, high pressure steam. The expander 12 is operated by the high temperature, high pressure steam generated by the evaporator 11 so as to generate mechanical energy. The condenser 13 cools decreased temperature, decreased pressure steam that has completed work in the expander 12 so as to turn it back into water. The water supply pump 14 pressurizes water discharged from the condenser 13, and re-supplies it to the evaporator 11.

As shown in FIG. 2, an open/close valve 15 for cutting off the supply of water is disposed between the evaporator 11 and the water supply pump 14, and an open/close valve 16 for cutting off the supply of steam is disposed between the evaporator 11 and the expander 12. Further, a motor/generator 17 is connected to the expander 12. The rotational speed of the expander 12 is controlled by regulating a load of the motor/generator 17. A Rankine controller Cr controls, based on a signal such as a fuel injection quantity Ti or an engine rotational speed Ne, the rotational speed of a motor 18 for driving the water supply pump 14, the load of the motor/generator 17, and opening/closing of the two open/close valves 15 and 16.

FIG. 3 and FIG. 4 show the arrangement of temperature control means 21 included in the Rankine controller Cr. The temperature control means 21 includes feedforward water supply amount calculation means 22, feedback water supply amount calculation means 23, water supply amount control changeover means 24, and rotational speed calculation means 25. The feedforward water supply amount calculation means 22 calculates a feedforward water supply amount for the evaporator 11 based on the engine rotational speed Ne, the fuel injection quantity Ti, and the exhaust gas temperature of the engine E. The feedback water supply amount calculation means 23 calculates a feedback water supply amount by multiplying a deviation of the steam temperature at the exit of the evaporator 11 from the target steam temperature at the entrance of the expander 12 by a predetermined gain.

The target steam temperature is determined as follows. That is, as shown in FIG. 8, the efficiency of the evaporator 11 and the efficiency of the expander 12 of the Rankine cycle system change according to the steam temperature; when the steam temperature increases, the efficiency of the evaporator decreases and the efficiency of the expander increases, whereas when the steam temperature decreases, the efficiency of the evaporator increases and the efficiency of the expander decreases. Therefore, there is an optimum steam temperature (a target temperature) at which the overall efficiency of the two becomes a maximum.

The water supply amount control changeover means 24 changes over between a state in which the water supply amount for the evaporator 11 is controlled using a value obtained by subtracting the feedback water supply amount from the feedforward water supply amount as a target water supply amount, and a state in which water supply is suspended (a state in which the target water supply amount is 0). That is, when the steam temperature increases after initial water supply to the evaporator 11 is started accompanying starting of running of the engine E, normal control is carried out so that the steam temperature coincides with a target temperature. When the fuel injection quantity Ti becomes 0 from this state and the engine E stops, the open/close valve 15 downstream of the water supply pump 14 is closed so as to suspend water supply to the evaporator 11. When the fuel injection quantity Ti becomes greater than 0 from the water supply suspension state and the engine E re-starts, control returns to using the target water supply amount obtained by subtracting the feedback water supply amount from the feedforward water supply amount.

The rotational speed calculation means 25 calculates a target rotational speed for the water supply pump 14 based on the target water supply amount outputted by the water supply amount control changeover means 24 and the steam pressure at the exit of the evaporator 11; and controls the rotational speed of the motor 18 for driving the water supply pump 14 so that it coincides with the target rotational speed.

FIG. 5 and FIG. 6 show the arrangement of pressure control means 26 included in the Rankine controller Cr. The pressure control means 26 includes feedforward rotational speed calculation means 27, feedback rotational speed calculation means 28, rotational speed control changeover means 29, and P1 feedback term calculation means 30. The feedforward rotational speed calculation means 27 calculates a feedforward rotational speed based on a target pressure of steam supplied to the expander 12, a commanded water supply amount, and a steam temperature at the entrance of the expander 12. The feedback rotational speed calculation means 28 calculates a feedback rotational speed by multiplying a deviation of the steam pressure at the entrance of the expander 12 from the target pressure for steam at the entrance of the expander 12 by a predetermined gain.

The target pressure is set by applying the energy (flow rate) and temperature of steam supplied from the evaporator 11 to the expander 12 to the map of FIG. 9. This target pressure corresponds to a steam pressure at which the expander 12 is operated at a maximum efficiency.

The rotational speed control changeover means 29 changes over between a state in which the rotational speed of the expander 12 is controlled using a value obtained by subtracting the feedback rotational speed from the feedforward rotational speed as the target rotational speed, and a state in which the expander 12 is stopped (a state in which the rotational speed is 0). That is, when the steam pressure increases after the expander 12 starts rotating accompanying starting of running of the engine E, normal control is carried out so that the steam pressure coincides with the target pressure. When the fuel injection quantity Ti becomes 0 from this state, the engine E stops, and the steam pressure decreases, the rotation of the expander 12 is stopped by increasing the load of the motor/generator 17 or the rotation of the expander 12 is stopped by closing the open/close valve 16. When the fuel injection quantity Ti becomes greater than 0 from the stopped state of the expander 12 and the engine E starts running, control returns to using the value obtained by subtracting the feedback rotational speed from the feedforward rotational speed as the target rotational speed.

The PI feedback term calculation means 30 calculates a target torque for the motor/generator 17 from a deviation of the rotational speed of the motor/generator 17 (that is, the rotational speed of the expander 12) from the target rotational speed outputted by the rotational speed control changeover means 29, and the rotational speed of the expander 12 is feedback controlled at the target rotational speed by making the motor/generator 17 generate the above target torque as the load.

As shown in FIG. 7, in accordance with the control of the present embodiment explained above, since when the engine E stops, water supply to the evaporator 11 is instantaneously suspended (ref. region f), the temperature of steam generated by the evaporator 11 only decreases slightly from the target temperature (ref. region e), so that degradation of the operating efficiency of the expander 12 can be minimized. Furthermore, since when the engine E stops, rotation of the expander 12 rapidly decreases (ref. region h), the steam pressure only decreases slightly from the target pressure (ref. region g), so that degradation of the operating efficiency of the expander 12 can be minimized. In this process, if the expander 12 is instantaneously stopped, since there is a possibility that the steam pressure exceeds the target pressure, the rotational speed of the expander 12 is decreased as quickly as possible, after the steam pressure starts decreasing, within a range in which the steam pressure does not exceed the target pressure.

Moreover, when the engine output returns to its original state, since the water supply amount and the rotational speed of the expander 12 are increased from the moment of return, the steam temperature and the steam pressure can quickly converge on the target temperature and the target pressure.

Although one embodiment of the present invention has been explained above, the present invention can be modified in a variety of ways as long as the modifications do not depart from the subject matter of the present invention.

For example, in the embodiment, control when the engine E recovers after being stopped is explained, but the present invention may be applied to control when the engine recovers after being in an idling state. 

1. A Rankine cycle system comprising: an evaporator for heating a liquid-phase working medium with thermal energy of exhaust gas of an engine so as to generate a gas-phase working medium; a displacement type expander for converting the thermal energy of the gas-phase working medium generated by the evaporator into mechanical energy; temperature control means for manipulating an amount of liquid-phase working medium supplied to the evaporator so that the temperature of the gas-phase working medium supplied from the evaporator to the expander coincides with a target temperature; and pressure control means for manipulating the rotational speed of the expander by changing a load of the expander so that the pressure of the gas-phase working medium supplied from the evaporator to the expander coincides with a target pressure; wherein, simultaneously with operating conditions of the engine shifting to a fuel supply suspension state or a low load state, the temperature control means suspends supply of the liquid-phase working medium to the evaporator, and the pressure control means decreases the rotational speed of the expander as quickly as possible within a range in which the pressure of the gas-phase working medium does not exceed the target pressure.
 2. The Rankine cycle system according to claim 1 wherein, simultaneously with recovery of the operating conditions of the engine from the fuel supply suspension state or the low load state, the temperature control means increases supply of the liquid-phase working medium to the evaporator, and the pressure control means increases the rotational speed of the expander. 